OBD-II Code P0629 — What It Means & How to Fix It

What Does This Error Mean?

P0629 means the fuel pump control circuit is reading a voltage that is too high. When the ECM monitors the fuel pump control line and sees excessive voltage, it sets this code. This typically means the control wire has picked up voltage from another source, or the circuit is open and the voltage is floating high. Fuel pump operation may be erratic or uncontrolled. This requires prompt diagnosis since uncontrolled fuel pump operation is a fire risk.

Affected Models

All vehicles 1996+
Common in GM cars and trucks
Common in Ford vehicles
Common in Chrysler, Dodge, and Jeep vehicles
Common in vehicles with electronic fuel pump control modules

Common Causes

Open circuit on the fuel pump control wire causing voltage to float high
Short to voltage on the fuel pump control wire — wire contacting a powered circuit
Faulty fuel pump relay with internal damage allowing voltage bleed-through
Corroded wiring harness with damaged insulation near power wires
ECM output failure with the driver stuck in a high state

How to Fix It

Remove the fuel pump relay. With the relay removed and key on, check for voltage at the control wire pin in the relay socket. Voltage present here with the relay removed indicates the control wire is shorted to a power source.

If voltage is only present when the key is in the Start position, that is a different circuit and may be normal.
Inspect the fuel pump control wiring for contact with power wires. Look for areas where insulation has melted or worn through against a 12V circuit.

The control wire is typically a smaller wire that routes alongside the main wiring harness.
Check the relay itself for internal damage. With the relay unplugged from the circuit, measure resistance between all terminal combinations. Any unexpected continuity indicates an internal short.

Even if the relay looks fine externally, internal contacts can short after years of use.
If the control wire has no short to voltage and the relay is good, the fault may be an open circuit with the signal floating high. Trace the control wire for any breaks, open splices, or disconnected connectors.

An open circuit (broken wire) can cause the voltage to float in an unexpected direction depending on the circuit design.
Repair the wiring fault or replace the relay as needed. Clear the code and verify normal fuel pump operation by checking that the pump runs briefly when the key is turned to On and that fuel pressure is in spec.

After repair, confirm the pump turns OFF when it should — an always-on pump can overheat and fail prematurely.

When to Call a Professional

Fuel pump circuit diagnosis involves tracing wiring and testing relays. Relays cost $10 to $25. Wiring repair at a shop costs $75 to $200 depending on the fault. If the ECM has failed, replacement costs $200 to $600 or more. Do not ignore this code — an uncontrolled fuel pump can be a fire risk.

Frequently Asked Questions

Can P0629 cause a fire?

Potentially yes, in extreme cases. An uncontrolled fuel pump that runs when the ignition is off can pressurize the fuel system abnormally. Combined with a fuel leak, this is a fire risk. This is why P0629 should be taken seriously and diagnosed promptly.

Will the car still run with P0629?

It depends on the severity of the fault. If the fuel pump control is erratic, the car may run roughly or stall intermittently. If the control circuit has completely malfunctioned, the pump may not operate correctly at all.

Is P0629 different from P0628?

Yes. P0628 is low voltage on the fuel pump control circuit — usually a short to ground. P0629 is high voltage — usually a short to power or an open circuit. The diagnosis approach is the same but you are looking for opposite types of wiring faults.